Abstract : Monte Carlo techniques were used in a study of three basically different approaches to simulating the gamma radiation environment near the air/ground interface due to fallout uniformly distributed on the ground surface. The energy and angular distribution of the photon flux at a receiver 3 feet above the ground due to a Co60 point isotropic source also three feet above the ground and at separation distances of 100 to 800 feet was computed, and the results bore little resemblance to those from a uniform fallout field. Results computed for a receiver position three feet above idealized Ce144, Cs137, and Co60 infinite plane sources on the ground indicated good simulation of the angular distribution of the gamma-ray dose and, except for energies greater than 1.25 Mev, reasonable simulation of the energy spectra from fallout. A third source arrangement, called the 'compact simulator', simulated the fallout gamma radiation environment equally well. In the compact simulator, an overhead slab of water acts as a reflector to simulate air scattering. (Author)